With Swift 5, the Package Manager has been updated to allow platform specifications for packages. All the examples I have seen use a predefined enum or static value (I am unsure which) to define the platforms, like this:
If you are targeting Swift 5 exclusively and don't want to support Swift 4.x, this is limiting because there is no defined value for iOS 12.2, which is required if you are not going to ship the Swift binaries in your library. All is not lost. If you look at the source the version can be specified as any 2 or 3 dot separated string. So you can do this:
Now you can build exclusively for iOS 12.2.